This invention relates generally to liquid transfer systems, and more particularly provides a liquid transfer valve of the rotary operating type, such as disclosed in the referenced U.S. Pat. No. 4,152,391, for measuring and dispensing precise microliter volumes of samples, and which is characterized by the establishment of a pair of measuring chambers in a series coupled relationship capable of providing simultaneously a pair of precisely measured, different liquid volumes and directing each to a pair of different predetermined locations, each along with a respective same volume of diluent.
The liquid transfer valve provided in the referenced U.S. Pat. No. 4,152,391 included internal segmenting passageway means and at least one external hollow loop of precise internal volume thereby to provide the different volumetric quantities of a single liquid sample for dilution, each sample quantity being directed, along with a common volume of diluent, simultaneously to different predetermined locations. The segmenting passageway means were provided in the center movable disc of the patented valve, and the external loop was connected directly to said center disc. External connections for feed of diluent and for coupling to lines leading to the predetermined locations were made to the outer, stationary portions of the valve. This construction required relatively large volumes, especially for the external loop as its length required passage through one of the stationary valve members of the patented valve assembly and this gave rise to considerable so-called dead space whose volume was useless and contributed to considerable waste of sample. In addition to the relatively high cost of manufacture, the complexities of system operation required many connection couplings resulting in possible trouble areas resulting in wear and/or leakage. Considerable demand also has been encountered for conservation of sample volumes required for the testing procedure.
Conventionally, the patented valves as used commercially commonly included the feature thereof providing a second external loop, also coupled to the center movable disc, which loop functions as a measuring chamber when a prediluted sample was employed. Accordingly, not one but two external loops were required in the valve assembly to cover the possibility of using a prediluted sample, even if rarely encountered. Clearly this added to the cost. Where prediluted samples were encountered, a section of the transfer valve was not employed and in fact, was blocked off. Nevertheless, this section was required to be provided.
Sample volumes required to make the required dilutions were much higher than the minimums needed to make dilutions required by the technologically improved analytical apparatus now available. The quantities of samples obtained are limited and often are minimal, many times in the microliter range. Smaller volume sample dilutions were capable of being handled successfully. Nevertheless, the structures of the available valve assemblies, including the patented reference valves, sharply limited possible reduction in the actual sample volumes required in order to make dilutions for testing.
Thus a need has arisen to provide relatively small precisely measured volumes of liquid sample even in the microliter range, so as to minimize actual raw sampling volume requirements. Fulfillment of such need should be accomplished without increasing the complexity either of fabrication or operation of the transfer valve assembly. Accuracy is paramount. Capability of substitution of the resulting valve assembly for already installed prior valve assemblies, i.e. retro-fit, is an important factor which must be considered in providing for microliter sampling capability. One way of fulfilling the aforesaid need would be to reduce considerably the volume of dead space within any valve assembly employed, but there is considerable difficulty in effecting such reduction with presently available valve assemblies.